This invention relates to electrostatic printing devices and more particularly to duplex printing utilizing non-impact printing devices which utilize electronically addressable printheads for depositing developer in image configuration on plain paper substrates.
Many methods for automatiac duplex printing in xerographic processors are known. There is the two pass method employed in the Xerox 4000.TM. and 9400.TM. reproduction machines. That is, after the first side of copy sheets are imaged and fused, the sheets are collected in a duplex tray. Afther the last sheet in a set has been received in the duplex tray, the sheets are again passed through the xerographic processing stations. This time an image is transferred and fused onto the opposite side of each copy sheet having an image on the first side.
In the Xerox 9700.TM. machine, the copy sheets also pass through the processing stations twice. However, they are not collected in a duplex tray. After the first image has been transferred and fused, the sheets pass through a stop and reverse mechanism (inverter). Then the sheets join in an interleafing fashion the stream of copy sheets to receive an image on the opposite side.
There are some disadvantages with these systems, in particular for a given image throughput rate. For example, two passes through the fuser require more energy, and the fuser needs to operate at twice the speed. During the first pass through the fuser, the paper loses 50 percent of its moisture. This curls the paper and makes the second pass for duplexing difficult. Paper picks up oil on the first pass through the fuser, sometimes leading to image deletions on the second image and oil deposits on the photoreceptor. Jam rates during two-pass duplex operation are much greater than for simplex operation. In the first place, in a two-pass duplex system, the paper path is usually very long, and the paper has to negotiate all obstacles twice. Excessive paper curl is not only troublesome in the processor but also extremely difficult to handle in output stackers and finishing devices.
In other prior art systems such as in U.S. Pat. No. 4,095,979means are shown for "immediate" or single-pass duplex copying or forming first and second images sequentially on a photoreceptor. The first image is transferred from the photoreceptor to the first side of a copy sheet. Then the sheet is stripped off the photoreceptor, inverted while the first image remains unfixed, and then the second image is transferred to the second side of the copy sheet. Both images are then fixed onto the copy sheet by a suitable fuser. This type of system can be described as a "single-pass" to the fuser.
Other single-pass duplex printing methods use intermediate image carriers (belt or drum). The first and second images are sequentially formed on a photoreceptor. The first image is transferred to an intermediate image carrier. The copy sheet is then passed between the photoreceptor and the intermediate image carrier, simultaneously receiving first and second images.
The duplex methods discussed above only utilize one photoreceptor. Other systems, e.g. U.S. Pat. No. 3,580,070 and 3,775,102 deal with "single-pass duplex" methods employing two photoreceptors and two exposure systems. First images are deposited on one photoreceptor and second images are deposited on the other photoreceptor. These systems are considered th eultimate duplex throughput systems since they produce twice the number of images of "two-pass duplex" systems at same process speed. These "single-pass duplex" systems, however, generally require web paper feed in which the copy is spooled up on a roll or cut into individual sheets after fusing. This unfortunately, introduces additional components and complexity into the system.
U.S. Pat. No. 4,427,285 discloses a discrete copy sheet feed system rather than a web paper feed system. A two photoreceptor, "single-pass duplex" apparatus is disclosed wherein two images are formed, one on each photoreceptor and then transferred to opposite sides of the image receiving sheet.
U.S. Pat. No. 4,714,939 discloses an electrographic reproduction apparatus, of the single-pass type, capable of producing simplex or duplex copies on a receiver sheet traveling in a continuous direction along a path. The reproduction apparatus comprises a first dielectric member movable along a first path, a portion of such first path being tangent to and on one side of the sheet travel path. Transferable images, corresponding to information to be reproduced, are sequentially formed on such first member. A second member is movable along a second path. One portion of such second path is tangent to the sheet travel path on the opposite side from the first path and another portion of the second path, spaced from such one portion, is located to position the second member in image transfer relation to the first dielectric member. An electrostatic field, reversible in its effective direction, is utilized to transfer a transferable image from the first dielectric member to the second member at the portion of the second path where the first and second members are in image transfer relation and transfer such image from the second member to one side of a receiver sheet traveling along its travel path at the location where the position of the first path is tangent to the sheet travel path and for producing a duplex copy, a second image is transferred from the first dielectric member to the opposite side of such receiver sheet at the location where the portion of the first path is tangent to the sheet travel path.
U.S. Pat. No. 32,422 discloses a method and apparatus for producing duplex copies. First and second unfixed images are transferred to opposite sides of a copy sheet before fixing of either image to the copy sheet. The first and second unfixed images may be electroscopic images sequentially formed on a photoconductor by electrophotographic techniques. The first unfixed electroscopic image is transferred from the photoconductor to a first side of a copy sheet, the sheet is inverted while the first image thereon remains unfixed, the second unfixed electroscopic image is transferred to the second side of the copy sheet, the copy sheet with the first and second unfixed images thereon is then transported to a fixing station.
Another technique involving the use of only one photoconductor, utilizes an intermediate image transfer member to receive the first image formed on the photoconductor before transfer to a final support medium. The intermediate transfer member as disclosed in U.S. Pat. No. 3,671,118 and 3,697,170 is such a belt.
Of the various electrostatic printing techniques, the most familiar and widely utilized is that of xerography wherein latent electrostatic images formed on a charge retentive surface are developed by a suitable toner material to render the images visible, the images being subsequently transferred to plain paper.
A lesser known form of electrostatic printing is one that has come to be known as Direct Electrostatic Printing (DEP). This form of printing differs from the aforementioned xerographic form, in that, the toner or developing material is deposited directly onto a plain (i.e. not specially treated) substrate in image configuration. This type of printing device is disclosed in U.S. Pat. No. 3,689,935 issued Sept. 5, 1972 to Gerald L. Pressman et al. In general, this type of printing device uses electrostatic fields associated with addressable electrodes for allowing passage of developer material through selected apertures in a printhead structure. Additionally, electrostatic fields are used for attracting developer material to an imaging substrate in image configuration.
Pressman et al disclose an electrostatic line printer incorporating a multilayered particle modulator or printhead comprising a layer of insulating material, a continuous layer of conducting material on one side of the insulating layer and a segmented layer of conducting material on the other side of the insulating layer. At least one row of apertures is formed through the multilayered particle modulator. Each segment of the segmented layer of the conductive material is formed around a portion of an aperture and is insulatively isolated from every other segment of the segmented conductive layer. Selected potentials are applied to each of the segments of the segmented conductive layers which a fixed potential is applied to the continuous conductive layer. An overall applied field projects charged particles through the row of apertures of the particle modulator and the density of the particle stream is modulated according to the pattern of potentials applied to the segments of the segmented conductive layer. The modulated stream of charged particles impinge upon a print-receiving medium interposed in the modulated particle stream and translated relative to the particle modulator to provide line-by-line scan printing. In the Pressman et al device the supply of the toner to the control member is not uniformly effected and irregularities are liable to occur in the image on the image receiving member. High-speed recording is difficult and moreover, the openings in the printhead are liable to be clogged by the toner.
U.S. Pat. No. 4,491,855 issued on Jan. 1, 1985 in the name of Fuji et al discloses a method and apparatus utilizing a controller having a plurality of openings or slit-like openings to control the passage of charged particles and to record a visible image of charged particles directly on an image receiving member. Specifically, disclosed therein is an improved device for supplying the charged particles to a control electrode that has allegedly made high-speed and stable recording possible. The improvement in Fuji et al lies in that the charged particles are supported on a supporting member and an alternating electric field is applied between the supporting member and the control electrode. Fuji et al purports to obviate at least some of the problems noted above with respect to Pressman et al. Thus, Fuji et al alleges that their device makes it possible to sufficiently supply the charged particles to the control electrode without scattering them.
U.S. Pat. No. 4,568,955 issued on Feb. 4, 1986 to Hosoya et al discloses a recording apparatus wherein a visible image based on image information is formed on an ordinary sheet by a developer. The recording apparatus comprises a developing roller spaced at a predetermined distance from and facing the ordinary sheet and carrying the developer thereon. It further comprises a plurality of addressable recording electrodes and corresponding signal sources connected thereto for attracting the developer on the developing roller to the ordinary sheet by generating an electric field between the ordinary sheet and the developing roller according to the image information. A plurality of mutually insulated electrodes are provided on the developing roller and extend therefrom in one direction. A.C. and D.C. voltage sources are connected to the electrodes, for generating alternating electric fringte fields between adjacent ones of the electrodes to cause oscillations of the developer positioned between the adjacent electrodes along electric lines of force therebetween to thereby liberate the developeer from the developing roller.
U.S. Pat. No. 4,912,489 discloses a Direct Electrostatic Printing device comprising a printhead structure comprising a shield electrode structure and a control electrode structure supported by an insulative support member. The printhead structure is positioned such that the control electrode is opposite the toner supply. Wrong sign toner accumulates on the control electrode.
Single-pass duplex printing systems can offer significant reduction in paper handling complexity and greatly increase duplex output productivity. With low cost, high reliability marking approaches such as DEP printers, the addition of a second marking stage to allow single-pass duplex printing is justified because the extra machine complexity normally added for high throughput duplex (duplex path hardware, timing gates, timing sensors, etc.) would be eliminated.